Boris Johnson's previous remarks about about black and ethnic minority people were "deeply offensive", Wales' health minister has said.
Labour's Vaughan Gething said it was an "issue" having a prime minister who had used terms such as "watermelon smiles".
Mr Johnson has previously apologised for the comments made when he was a journalist back in 2002.
Justice Secretary Robert Buckland said he judged Mr Johnson on his actions as an elected politician.
During the BBC's Question Time, Mr Gething said the prime minister's comments, made in an article he wrote for the Daily Telegraph, were "deeply offensive".
Writing in 2002, before then-Prime Minister Tony Blair visited Democratic Republic of Congo, Mr Johnson said: "What a relief it must be for Blair to get out of England. It is said that the Queen has come to love the Commonwealth, partly because it supplies her with regular cheering crowds of flag-waving piccaninnies."
In the article, he added: "No doubt the AK47s will fall silent, and the pangas will stop their hacking of human flesh, and the tribal warriors will all break out in watermelon smiles to see the big white chief touch down in his big white British taxpayer-funded bird."
Mr Johnson apologised for the comments in 2008, during his successful campaign to be mayor of London.
Mr Gething also criticised Plaid Cymru leader Adam Price's comments, "comparing the experience of Wales with colonialism", when he called for reparations last year.
"It is a real issue. For people who look like me, having a prime minister who used language like watermelon smiles and piccaninnies, it mattered," he said.
"Just as someone comparing the experience of Wales to colonialism and the African American experience, it matters and it's offensive".
Mr Buckland said he judged Mr Johnson on his actions as an elected politician, not on something he wrote 15 years ago.
The Conservative MP said: "I would say that journalists write lots of things, write lots of polemic and lots of things they would later come to regret or later choose to regret.
"I can't speak for him about something that might have been written 15 or so years ago. I've got to judge a person on his actions as an elected politician."
At a party conference in October 2019, Mr Price said Wales was owed "reparation for a century of neglect that has left a country, rich in its resources, a bitter legacy of poverty, sickness, blighted lives and broken dreams".
He was criticised and the issue came up again in a later interview during the general election campaign, when Mr Price said Wales had an "extractive economy" and "political power centre outside of our nation".
"For most people that is analogous, if not identical, to the experience of colonialism," he said.
Plaid Cymru later conceded Mr Price was "wrong" to call for reparations for Wales without referring to the country's role in empire.
Responding to Mr Gething's comments, Plaid MP Liz Saville Roberts said: "When we are looking at the real big issues in the UK, one of the petty things we do is bring it down to personalities."
She said Mr Price had called for an "independent inquiry into structural racism in Wales" and was leading calls for a Welsh BAME museum.